Taiwanese utility model patent no. M487160 discloses a conventional headstock assembly 9 for a threading machine (see FIG. 1) in which a headstock 91 is slidably mounted on a guide rail unit 90 in a linear direction, a spindle shaft 92 is rotatably mounted in the headstock 91, and a chuck member 93 for gripping a workpiece (W) is coupled to a right end of the spindle shaft 92 so as to rotate with the spindle shaft 92. In the conventional headstock assembly, the linear motion of the headstock 91 is driven by a drive member 94, and the rotary motion of the spindle shaft 92 is driven by another drive member (not shown).